1. Field of the Invention
The present invention relates to an image forming apparatus, and a carrier, a toner and a developer used therein.
2. Discussion of the Background
Image forming methods such as electrophotography, electrostatic recording and electrostatic printing include, e.g., adhering a toner included in a developer to an image bearer such as a photoreceptor, on which a latent image is formed, to form a toner image thereon in a developing process; transferring the toner image onto a transfer medium such as a transfer paper in a transfer process; and fixing the toner image thereon in a fixing process. The developer includes a two-component developer including a magnetic carrier and a toner, and a one-component developer (magnetic or nonmagnetic toner) which does not need a carrier.
A dry toner prepared by the following method has conventionally been used. The method includes melting, kneading upon application of heat, cooling and hardening a toner binder such as a styrene resin and a polyester resin with a colorant, to prepare a cooled and hardened mixture; and pulverizing the cooled and hardened mixture. However, demands for higher-quality images are increasing recently, and particularly, toners are having smaller particle diameters and higher sphericity to realize production of high-definition color images. Toners having smaller particle diameters have better dot reproducibility, and toners having higher sphericity improve their developability and transferability.
The toner formed of a resin has a volume-average particle diameter of about 10 μm, and an inorganic/organic particulate material is externally added thereto to give fluidity thereto for covering the transportability and mixability thereof. Inorganic particulate materials such as silica, alumina and titanium oxide are typically attached to a toner by dry mixing and stirring them with the toner with a mixer.
However, the inorganic particulate material externally added to a toner is soon buried therein by a stress with a member (of an apparatus) contacting thereto, resulting in deterioration of fluidity, feedability, developability and chargeability thereof. In addition, the inorganic particulate material not fully mixed therewith is released therefrom and floats in the apparatus to contaminate the apparatus, and further adheres to an image bearer such as a photoreceptor in the apparatus and becomes a base that toner fixes to, resulting in filming of the toner.
Recently, a number of methods granulating toner particles in a liquid, such as a suspension polymerization method, an emulsification polymerization method and a dispersion polymerization method, have been developed to make a toner have smaller particle diameter and more sphericity. However, a spherical toner easily rolls on an image bearer and passes through a cleaning blade formed of an elastomer, resulting in poor cleaning.
Japanese Laid-Open Patent Publications Nos. 2002-244314 and 2002-351129 disclose a toner including an additive, the release rate of which is determined by a particle analyzer. However, the particle analyzer does not have enough detection sensitivity to detect a slight difference between the adherence amount and release amount of the additive. In addition, the particle analyzer is not capable of designing a suitable control range of an additive having a low adherence rate of its parent particles, such as silica having a large particle diameter.
Japanese Patent No. 3129074 and Japanese Laid-Open Patent Publication No. 2000-122336 disclose a toner including an additive evaluated by an ultrasonic homogenizer. This has an effect on the chargeability of a toner and filming reduction thereof over a photoreceptor, but does not improve both the filming reduction and cleanability thereof.
One of the critical elements influencing a life span of a two-component developer using a magnetic carrier is adherence of toner constituents (i.e., spent toner constituents) to the carrier. Such toner constituents include a resin, a release agent (wax), a charge controlling agent and an external additive, which deteriorate chargeability of the developer when attached to the carrier. However, the influence that the severity, adherability and adherence status of each constituent has on the carrier are unknown.
When a toner receives a stress from, e.g., a developing screw in an image developer and loses fluidity, the toner and a carrier are not well mixed and the developer is not uniformly charged, thus resulting in production of foggy images. Particularly, in a low-temperature and low-humidity environment, when a developer increases its average charge quantity and the charge quantity between the charged toner and uncharged toner becomes large, the fluidity of the developer deteriorates, which is assumed to cause production of foggy images.
Because of these reasons, a need exists for an image forming apparatus capable of reliably producing high-quality images, using a long-life developer having much less spent toner and capable of reducing production of foggy images in a low-temperature and low-humidity environment.